Devices which create sub-harmonic tones in response to the tone signal of a musical instrument are known in the prior art.
U.S. Pat. No
U.S. Pat. No. 3,535,969 issued to Bunger describes one such deivce which uses transmission gates each controlled by a frequency divider to produce sub-harmonic voices which are accurately proportional in amplitude and in tone color to those of the incoming tone signal.
In such a device, the phase relationship between the tone signal being gated and the control voltage from the frequency divider is assumed to be rather constant.
A problem exists when a tone signal from a transducer monitoring the displacements of a bowed vibrating element in and about the plane of bowing is gated to produce a sub-harmonic tone, that the tone color of the sub-harmonic tone will change upon a change in the direction of bowing although the tone color of the signal from the transducer remains constant for both bowing directions.
This undesirable change in the tone color of the subharmonic tone signal occurs because the phase relationship between the tone signal from the transducer and the control signal driving the gate changes upon a change in the direction of bowing.
The waveform of the tone signal from the transcducer is asymmetrical and reverses when the direction of bowing is reversed. When the transducer signal is applied to a comparator to produce a square wave at the frequency of the note being played, the pulse width of the resulting square wave will change significantly upon a change in the direction of bowing, causing a corresponding time shift in the triggering of the divider.
A second problem exists when the sub-harmonic tone signals are partially formed of out-of-phase portions of the transducer signal, that when such a sub-harmonic tone signal is mixed with the corresponding transducer signal, the out-of-phase portions of the sub-harmonic tone signal tend to cancel the corresponding in-phase portions of the transducer signal, thus partially defeating the efforts to produce a sub-harmonic signal in that manner.
A third problem exists if the signal gates are left in a state of conduction when the fundamental detector is inactive, that the entire transducer signal will be passed when the level of a decaying tone becomes lower than that required to activate the fundamental detector. This problem is much more noticeable with bowed instruments than it is with wind instruments.
It is therefore a first object of the present invention to provide an improvement in sub-harmonic tone generators of the signal gating type, whereby the tone color of the resulting subharmonic tone signal is independent of the direction of bowing.
It is a second object of the present invention to produce sub-harmonic tone signals which can be mixed in any ratio with the tone signal from the originating transducer and also with other sub-harmonic tone signals from the same transducer signal without causing any cancellation between them.
It is a third object of the present invention to prevent the passing of any portion of the transducer signal through the sub-harmonic generating signal gates in the absence of detection of the fundamental frequency of the played note.
It is a fourth object of the present invention to produce sub-harmonic tone signals, the waveforms of which closely approximate those of the corresponding bowed instruments of the same frequency range.